This invention relates to improvements in the manufacture of a coaxial cable in which a rib or tape of a synthetic resin is spirally wound on the outer peripheral surface of an inner conductor and then an outer sheath layer or tubular member made of a synthetic resin is extruded thereon so as to form an outer insulating layer or sheath which is integral with the inner conductor and the tape wound thereon.
FIG. 1 illustrates in cross-section a coaxial cable of an insulating sheath type. Reference numeral 1 designates an inner conductor made of a copper or aluminum tube or the like, and reference numeral 2 designates a synthetic resin rib or tape having a rectangular cross-section. Rib 2 is wound on the outer peripheral surface of inner conductor 1. Reference numeral 3 designates a tubular synthetic resin which is extruded around the rib 2 so as to be integral with the rib 2 along the length of the inner conductor 1. This construction produces an insulating layer which is composed of the tubular synthetic resin 3 and the synthetic resin rib or tape 2. Formed around the outer surface of the tubular synthetic resin 3 is an outer conductor 4 and a synthetic resin sheath 5.
A typical prior art process for manufacturing coaxial cables having the configuration described above includes the step of extruding a synthetic resin material 3 from an extruder around the outer peripheral surface of an inner conductor 1 having a synthetic rib or tape 2 spirally wound thereon, followed by the step of cooling and solidifying of the material 3 so as to form the insulating layer. However, in order to feed the unextruded inner conductor into an extruder, the extruded and cooled coaxial cable core is pulled by a drive means provided on the side of a take-up means.
The prior art process discussed above suffers from several major deficiencies. Because the coaxial cable core that is cooled and solidified is subjected to a tension by the drive means, a lack of uniformity results in the construction of the coaxial cable core. Furthermore, an uneven outer peripheral surface of the tubular layer or member having a spiral concave pattern occurs during the cooling and solidification of the insulating layer because the thermal contraction of the portions of the tubular layer which contact with the rib or tape is relatively larger than the thermal contraction of the non-contacting portions. As a result, the outer diameter of the tubular layer does not exhibit an uniformity along the length thereof. In other words, a spiral recessed portion is created along the spiral rib. The aforenoted deficiencies produce an increased variation in electrostatic capacity of a coaxial cable core the length thereof, which has a detrimental influence on the electrical characteristics of a coaxial cable.